The present invention relates to a new and improved method of eliminating organic and inorganic bound nitrogen from domestic and industrial waste water.
There is already known to the art a process for the biological purification of waste water or sewage according to the activated sludge technique in which decomposition of the wastes or contaminants is carried out, on the one hand, by bacteria and the removal of the increased bacteria by bacteria-consuming organisms, in two successive completely separate stages. In so doing, the waste water or sewage delivered to the first stage, if desired mechanically pre-clarified, is aerated in an aeration tank or vessel while providing optimum conditions for the life of the bacteria. The concentration of oxygen is chosen such that the clarified or purified discharge of a settling tank or basin associated with the first stage and following the aeration tank and into which following aeration there is fed the sewage, after a certain residence time, no longer contains any free oxygen. This sewage which is free of oxygen and removed from the settling basin is now again aerated in an aeration tank at the second stage and the sludge concentration is regulated such that the discharge from a settling basin associated with the second stage and arranged after the aeration tank of the second stage, after a certain residence time, possesses an oxygen content of at least 2 mg/l.
Owing to the deposit of domestic and industrial sewage into natural waters its content of ammonium- and nitrate nitrogen compounds is continuously increased. Ammonium constitutes a danger for such natural waters since with increased pH-values such is toxic to fish or with nitrification can lead to oxygen depletion of the water. On the other hand, the nitrate acts as a fertilizer and causes increasing eutrophication of the water. Furthermore, it renders more difficult or endangers processing of such water for drinking purposes, since nitrates when present in too great a concentration can lead to possible health hazards to human and animal life. The high ammonia and nitrate content of natural water is also, among other things, brought about by an insufficient elimination of these substances by previously known conventional biological waste water treatment techniques and processes, inclusive of the aforementioned process.
Hence there is present a real need for waste water treatment techniques and processes capable of eliminating to the greatest possible extent ammonium and nitrate substances as well as organically bound nitrogen. Different proposals aim at nitrifying in known manner the ammonium in an activated sludge process and to de-nitrify the nitrate-enriched effluent in a special subsequent stage. During nitrification and de-nitrification the following is to be observed:
With nitrification in an activated sludge system ammonia nitrogen substances are microbiologically oxidized into nitrites and nitrates. A large proportion of the organic bound nitrogen which cannot be absorbed or otherwise consumed is transformed into ammonia-nitrogen by ammonifying bacteria. The conversion of ammonia to nitrite is effected by means of "nitrosomonas" bacteria, the conversion of nitrite into nitrate is effected by the "nitrobacter". EQU NH.sub.4 .sup.O.spsb.2 NO.sub.2.sup.- .sup.O.spsb.2 NO.sub.3.sup.-
it is presumed that the growth rate of nitrosomonas is smaller than nitrobacter so that the former presents a limiting factor for nitrification. Whether nitrification is possible in an activated sludge installation is dependent upon different factors, among others, temperature, aeration time, sludge concentration, sludge load, oxygen content and inhibitors such as the pH-value.
Nitrification is basically only then possible if under certain conditions a certain sludge load is not exceeded. With sludge loads exceeding 0.2 kg BOD.sub.5 /kg DS (wherein the symbol BOD.sub.5 represents the biological oxygen demand for 5 days and symbol DS signifies dry solids) at a temperature range below 15.degree. C there does not occur any or only a very unsatisfactory nitrification. At 20.degree. C and a sludge load up to 0.33 kg BOD.sub.5 /kg DS it is possible to obtain complete nitrification.
The presence of different toxic substances in the sewage can hinder nitrification. In particular, thiourea, cyanide, phenols and heavy metal salts can inhibit the growth of nitrifying bacteria.
During denitrification nitrite and nitrate are reduced into elementary nitrogen and nitrous oxide. This can be expressed by the following equation. EQU NO.sub.2 .sup.- + NO.sub.3 .sup.- .fwdarw. N.sub.2 + N.sub.2 O
the mechanism of denitrification has not been yet fully clarified. Generally, this process is considered as a strictly anaerobic process. Basically hydrogen donors appear to be necessary for denitrification. Apart from the influence of the oxygen content during denitrification the pH-value and the residence time plays a role. There has been noticed a correlation between the pH-value and the influence of the oxygen potential upon the denitrification rate. At a pH-value exceeding 7 denitrification in the presence of oxygen is totally inhibited.
In summation it is to be mentioned that with all trials undertaken there could be determined the interrelationship between sludge concentration and time as well as the temperature and sludge load during nitrification. Similarly it was recognized that denitrification is basically possible.